In private automatic mobile radio networks or trunked networks, several user groups or even several user organizations share the same radio channels and other resources in a common network. Typical private radio network applications include energy production and distribution, communal and municipal services, transportation, construction, maintenance, and emergency services, such as police, fire alarm and ambulance. A private mobile radio network may offer fast access within the user's own organization, individual or group communication, data communication, etc. The system checks the subscriber authorization and allocates resources. An individual subscriber is unaware that he uses the same frequencies as other subscribers as the system automatically selects a free radio channel for a call during the call set-up. This kind of radio telephone systems usually have a rather limited Geographical coverage, and so the user may easily get outside the service area of his own system, and thus cannot use his radio telephone. Therefore there has arisen a need to be able to use the same radio telephone in several different systems.
The specifications MPT 1327 and MPT 1343 issued by the British Ministry of Trade and Industry have become kind of de facto standards in Europe for private mobile radio networks. MPT 1327 specifies signalling over the radio path, while MPT 1343 specifies the operation and structure of the radio telephone. Systems complying with these specifications have been and will be introduced in different European countries. All radio telephones used in the different systems actually comply with the same MPT 1343 specification, wherefore they can, in principle, be transferred from one radio telephone network to another by varying the network-specific radio telephone parameters complying with the MPT 1343.
The radio telephone systems usually employ special radio channel numbers in all internal signalling for representing transmitting and receiving frequencies, i.e. radio channels, each channel number representing a certain pair of transmitting and receiving frequencies. A radio telephone or a similar radio device used in this kind of system converts each channel number internally into corresponding frequency synthesizer control data which controls the frequency synthesizer so as to cause it to be locked to the transmitting and receiving frequency pair corresponding to the channel number in the current radio system. When using the same radio telephone in different radio telephone networks, problems may be caused by the fact that a certain channel number, e.g. channel 1, may correspond to different frequencies in different radio telephone systems, as illustrated in FIG. 1. In FIG. 1, the frequency of the channel CHO of the radio telephone system A is 410 MHz, while higher frequencies at intervals of 12.5 kHz or 25 kHz correspond to the other channel numbers. The channel numbering of the system B in turn is such that the frequency corresponding to the channel CHO is 420 MHz. As a consequence, the radio telephone cannot operate in both systems A and B as the channel numberings and frequency bands of the systems do not match.